1. Field of the Invention
The present invention relates to gas-sensitive sensors, and more particularly to a metal oxide semiconductor field effect transistor's (MOSFET's) FET type gas-sensitive device having a horizontal floating gate for solving problems of the conventional sensor having a vertically formed gate and sensing layer and for being used in fabricating chemical sensors and biosensors etc.
2. Description of the Related Art
Recently, in order to sense chemical and bio materials, various structural sensors are being developed. Among these sensors, field effect transistors (FETs) having a floating gate are being studied due to having a high input impedance and a high amplification factor. Particularly, because of the needs of a low power, a high sensitivity induced by a high transconductance and a convergence fused with the conventional CMOS circuit, the research on FET type sensors is gradually increasing.
In the conventional study, it is mainly embodied to have a structure that a floating gate and a sensing layer are vertically formed. Main fabrication process of the conventional structures includes forming a gate insulating layer on a silicon substrate, forming a sensing layer on a top surface of the gate insulating layer and forming a control electrode on the sensing layer. The sensing principle is that, before and after sensing, some effects such as a capacitance change, a work function change and a charge generation/extinction appear in a sensing layer, some effects induce a potential change in a channel and, finally, the potential change induces a drain electric current change. The drain current change is read to judge gas-sensitivity and to quantitatively express the degree of sensing.
By reviewing the conventional publication documents cited below, we can see the following: U.S. Pat. No. 4,849,798 (hereinafter referred to as Patent document 1) shows a typical vertical shaped structure that includes a floating electrode formed on a gate insulating layer, a sensing layer on the top surface of the floating electrode and a control electrode on the top surface of the sensing layer. This structure is useful as a method for sensing a drain current by measuring a capacitance change of a sensing layer before and after sensing. In the device structure of Patent document 1, because the sensing layer is formed between the vertically formed insulator and control electrode, the reactants are permeated into a side wall to cause the reaction. But because a gate region (defining a width and a length of a channel) of FET used in the sensor has a large size generally, the reaction speed is slow. Thus, it takes too much time to react by a long permeated length from the side wall and it also spends a vast time in removing the permeated gas. For reducing the reaction time, the gate size must be decreased as much as possible. However, it needs expensive apparatuses to reduce the gate area in a current semiconductor process. Thus, it causes a cost increasing problem. In order to solve the problem, a structure using a porous metal as the control electrode to react gas particles to the sensing material via the control electrode and another structure using a control electrode as the sensing material are developed as gas sensors, but the kind of gas being able to sense is limited. Because only one material is used to form the insulating layer in the fabrication process, it is difficult to sense various gases. And this structure, as mentioned above, is a vertical shape using a capacitance change before and after sensing. Thus, if the sensing layer is an insulating material, the electrode can be formed on the sensing layer. If the sensing layer is a semiconductor material, it needs processes for forming an insulating layer and an electrode. In this case, it has a problem to cause physical and chemical damages on the sensing layer during the electrode and insulating layer forming processes. And most MOSFET type sensors are covered with a thick passivation layer for solving a reliability problem induced by the exterior environmental exposure. In Patent document 1, the passivation layer is formed just above the gate insulating layer to form a thick gate insulating layer. Thus, the coupling ratio of the control electrode is reduced and it comes to fall the sensitivity of the sensor.
In U.S. Pat. No. 7,772,617 (hereinafter referred to as Patent document 2), it shows a structure having an air layer between a sensing layer and a floating gate. In this structure, the sensing layer plays a role of a control electrode. After sensing, a work function change of the sensing layer and a FET threshold voltage is consequently changed to induce a drain current change as a principle of a sensor. This sensor structure has an air layer for solving the problem that the device reaction time is determined depending on the size of the gate described in Patent document 1. The air layer forming process includes forming a sacrificial layer between a floating gate and a sensing layer and forming the air layer by etching the sacrificial layer isotropically. However, because of the air layer with a small dielectric constant, a coupling ratio between the control electrode and the floating electrode is decreased and it comes to reduce the sensitivity of a sensor. In order to solve the problem, the area between the control electrode and the floating electrode has to be increased. But when the area is increased, there are other problems. First, when the area of a sensor is increased, the number of sensors fabricated on one substrate is reduced and the fabricating cost is increased. Second, the conventional MOSFET type gas sensors mainly use the sensing materials based on a metal oxide. The sensing materials are mainly operated in high temperature. If the area of a sensor broadens, the area of a sensing layer region also broadens. Because the more heat energy has to be transferred into the sensing layer, the electric power consumption is increased. Third, when the sacrificial layer is isotropically etched using an etching solution, there is a problem the insulating layer (i.e., passivation layer) protecting the sensing layer and the active region is exposed in the etching solution for a long time. Accordingly, there is a selection limit of materials for forming the sensing layer that does not nearly react with the etching solution removing the sacrificial layer. And a thick passivation layer has to be formed to protect the active region. Bur if the thickness of the passivation layer is increased, the coupling ratio between the control electrode and the floating electrode is reduced.
In addition, for fabricating the sensor of Patent document 2, at least 6 masks are needed and it makes to increase the process complexity and to generate a difficulty in the predetermined fabrication process step. Thus, the yield is reduced and the fabricating cost of a sensor is increased.
In the sensor introduced in a paper, Mario Alfredo Reyes Barranca et al., “Using a Floating-Gate MOS Transistor as a Transducer in a MEMS Gas Sensing System,” Sensors 2010, 10, 10413-10434 (hereinafter referred to as Non-patent document 1), when the sensing material reacts to gas, charges are induced in the floating electrode and have influence on a drain current. In this device structure, a sensing region and a control electrode are equipped together on a floating electrode and it is a sensor using the effect of a charge generation/extinction depending on before and after the gas reaction in a sensing material. In the structure, if a sensing material is reacted to gas, net charges are generated in the sensing material and the net charges induce again charges in the floating electrode located the below. The induced charges change the potential of the floating electrode and it comes to change a FET threshold voltage and to change a drain current. And under the sensing layer, a separate heater can be formed to transfer heat to the sensing layer. In this case, for protecting the heat transfer into an unwanted region, the silicon substrate is etched. In this structure, because the floating electrode must be widely formed, the parasite capacitance components are increased. And the important parts of the sensor can be damaged in the etching process of a silicon substrate.
Therefore, the development of devices having a new structure is needed to solve the problems in the conventional published devices.